I learned something else that's new about the porcupine quill: the tip is barbed so it can penetrate easily and make it so much harder to pull out.

Their quills easily penetrate the skin of a predator but are much harder to get out. And unlike the African porcupine, hedgehog, and echidna—which have smooth spines—the North American porcupine’s quills are tipped with microscopic backward-facing barbs that act like grappling hooks and lodge in the skin of a threat, Karp says. Each quill has up to 800 barbs, all of which are located on the first 4 mm of the quill’s tip. The quills easily detach from the porcupine, allowing it to escape while the sharp hair remains painfully lodged in the enemy.

“We were most surprised to find that the barbs not only increased the force required for removal, but reduced the penetration force," Karp says.

Read more: More Than Sharp: The Science of Porcupine Quills - Popular Mechanics

I've heard that porcupines can actually put their quills "down" (or maybe they only put them "up" when threatened.) So it is actually possible to pet a porcupine and not get quills stuck in you. Though there is always the danger of having it happen by accident.